Platelet arachidonate metabolism and platelet function in zinc-deficient rats

A zinc transporter, transmembrane protein 163, is critical for the biogenesis of platelet dense granules

Lysosome-related organelles (LROs) are a category of secretory organelles enriched with ions such as calcium, which are maintained by ion transporters or channels. Homeostasis of these ions is important for LRO biogenesis and secretion. Hermansky-Pudlak syndrome (HPS) is a recessive disorder with defects in multiple LROs, typically platelet dense granules (DGs) and melanosomes. However, the underlying mechanism of DG deficiency is largely unknown. Using quantitative proteomics, we identified a previously unreported platelet zinc transporter, transmembrane protein 163 (TMEM163), which was significantly reduced in BLOC-1 (Dtnbp1sdy and Pldnpa)-, BLOC-2 (Hps6ru)-, or AP-3 (Ap3b1pe)-deficient mice and HPS patients (HPS2, HPS3, HPS5, HPS6, or HPS9). We observed similar platelet DG defects and higher intracellular zinc accumulation in platelets of mice deficient in either TMEM163 or dysbindin (a BLOC-1 subunit). In addition, we discovered that BLOC-1 was required for the trafficking of TMEM163 to perinuclear DG and late endosome marker-positive compartments (likely DG precursors) in MEG-01 cells. Our results suggest that TMEM163 is critical for DG biogenesis and that BLOC-1 is required for the trafficking of TMEM163 to putative DG precursors. These new findings suggest that loss of TMEM163 function results in disruption of intracellular zinc homeostasis and provide insights into the pathogenesis of HPS or platelet storage pool deficiency.

The contribution of zinc to platelet behaviour during haemostasis and thrombosis

Platelets are the primary cellular determinants of haemostasis and pathological thrombus formation leading to myocardial infarction and stroke. Following vascular injury or atherosclerotic plaque rupture, platelets are recruited to sites of damage and undergo activation induced by a variety of soluble and/or insoluble agonists. Platelet activation is a multi-step process culminating in the formation of thrombi, which contribute to the haemostatic process. Zinc (Zn(2+)) is acknowledged as an important signalling molecule in a diverse range of cellular systems, however there is limited understanding of the influence of Zn(2+) on platelet behaviour during thrombus formation. This review evaluates the contributions of exogenous and intracellular Zn(2+) to platelet function and assesses the potential pathophysiological implications of Zn(2+) signalling. We also provide a speculative assessment of the mechanisms by which platelets could respond to changes in extracellular and intracellular Zn(2+) concentration.

Simultaneous measurement of prostaglandin and arachidonoyl CoA formed from arachidonic acid in rabbit kidney medulla microsomes: the roles of Zn2+ and Cu2+ as modulators of formation of the two products

Under physiological conditions, small amounts of free arachidonic acid (AA) is released from membrane phospholipids, and cyclooxygenase (COX) and acyl-CoA synthetase (ACS) act competitively on this fatty acid to form prostaglandins (PGs) and arachidonoyl-CoA (AA-CoA). To date, there is no information about the factors deciding the metabolic fate of free AA into these two pathways. In this study, we tried to establish a method for the simultaneous measurement of PG and AA-CoA synthesis from exogenous AA in microsomes from rabbit kidney medulla. The kidney medulla microsomes were incubated with [14C]-AA in 0.1 M-Tris/HCI buffer (pH 8.0) containing cofactors of COX (reduced glutathione and hydroquinone) and cofactors of ACS (ATP, MgCl2 and CoA). After incubation, PG (as total PGs), AA-CoA and residual AA were separated by selective extraction using petroleum ether and ethyl acetate. When 60 microM AA was used as the substrate, indomethacin (an inhibitor of COX) and triacsin C (an inhibitor of ACS) reduced only PG and AA-CoA formation, respectively. On the other hand, when 5 microM AA was used as the substrate, indomethacin and triacsin C came to increase significantly the AA-CoA and PG formation, respectively. Thus, the experiments utilizing indomethacin and triacsin C revealed that the incubation using 60 microM AA can simultaneously detect the changes in the activities of COX and ACS caused by drugs, while the incubation using 5 microM AA can detect the changes in the product formation elicited by the resulting shunt of AA. Further, using these incubation conditions, the effects of Zn2+ and Cu2+ on the PG and AA-CoA formation were examined. Zn2+ inhibited the AA-CoA synthesis from 60 microM AA without affecting the PG synthesis. In contrast, when 5 microM AA was used as the substrate, a significant increase in the PG formation was observed in the presence of this ion, indicating that drug actions on the PG formation from AA by the kidney medulla microsomes may change depending on the substrate concentration. On the other hand, Cu2+ increased PG synthesis and inhibited AA-CoA synthesis from both 60 and 5 microM AA. These results suggest that the simultaneous measurements of PG and AA-CoA formation by the kidney medulla microsomes under high (60 microM) and low (5 microM) substrate concentrations can investigate the direct and indirect actions of drugs on the COX and ACS activities, and are useful for clarifying the haemostatic control of the metabolic fate of AA into the two enzymatic pathways. Furthermore, this study showed that Zn2+ and Cu2+ can modulate PG and AA-CoA formation by affecting COX activity, ACS activity, and/or the AA flow into the two enzymatic pathways.

The effect of zinc deficiency and food restriction on prostaglandin E2 and thromboxane B2 in saliva and plasma of rats

Zinc has been implicated in the regulation of prostaglandins and other arachidonic acid derivatives. Studies of zinc-deficient animals, however, are compromised by concomitant reduction in food intake that may also alter eicosanoid levels in body tissues and fluids. In this study, three groups of rats, designated as zinc-deficient, pair-fed and control, were fed diets containing 1 ppm, 15 ppm (in amounts paired to deficient rats) and 15 ppm Zn ad libitum, respectively, for 6 weeks. Saliva and blood were analyzed for PGE2 and TXB2 by radioimmunoassay. Saliva concentrations of both eicosanoids were lower (p less than 0.05) in the pair-fed animals, but not significantly altered by zinc deficiency. Plasma levels of PGE2 and TXB2 were unchanged by either zinc deficiency or food restriction. The results of this study support the contention that the effect of zinc on these prostaglandins is not mediated by altered rates of synthesis or degradation but rather by effects on eicosanoid function.